On many occasions, a large-sized tire for trucks and buses used on unpaved roads, especially on rough roads where stones and rocks are exposed and an off-road tire used in a quarry, etc., are cut in the tread portion or the sidewall portions thereof. The cut growth results in tire destruction. The cut itself allows rain to penetrate to steel cords reinforcing the tire and to corrode them. Therefore, a rubber composition having good cut resistance is necessary for a large-sized tire and an off-road tire.
It is known to compound a large amount of carbon black in a rubber composition for the tire tread in order to improve cut resistance. However, this method results in bad chipping resistance on rough roads, high hysteresis loss and bad workability in the mixing and extruding processes.
U.S. Pat. No. 3,927,144 discloses that cut resistance is improved with a rubber composition comprising a styrene-butadiene rubber and dicyclopentadiene resin having a softening point of about 50-200 degrees C. and a bromine number of 40-150. However, the use of dicyclopentadiene resin is unsatisfactory to achieve good cut resistance in a styrene-butadiene rubber composition and does not improve the cut resistance of a natural rubber composition which is used for a large-sized tire because of the low hysteresis loss. Accordingly, a need for a new and inexpensive rubber additive continues.